Mass-Thickness and Bragg Contrast

The interaction of electrons with heavy atoms is stronger than with light atoms. If the thickness is homogeneous, areas in which heavy atoms are concentrated appear with darker contrast than such with light atoms (mass contrast). Of course, more electrons are scattered in thick than in thin areas; thus, thick areas appear dark (thickness contrast). However, a thick area with light elements might have the same contrast as a thinner area with heavier atoms. If the sample has crystalline areas, many electrons are strongly scattered by Bragg diffraction (especially if the crystal is oriented along a zone axis with low indices), and this area appears with dark contrast in the BF image as well (diffraction contrast). To summarize, independent of the reason for its scattering, all scattered electron beams are deflected away from the optical axis and blocked by the objective aperture, and thus the corresponding areas appear dark in the BF-image. Generally, mass-thickness contrast and Bragg contrast are important in bright and dark field imaging.

Contrast generation in BF mode (only the direct beam contributes to the image). Areas that contain heavy atoms or are thick appear dark.

ETH Zürich | ETH chemistry department | ETH inorganic chemistry